We present a novel ONU architecture for coherent UDWDM-PON with randomly distributed ONU wavelengths. The ONU is built with one or two lasers exhibiting simple limited thermal tunability in a specific optical band, where the wavelengths are managed in a Statistical UDWDM approach. If only one laser is present at the ONU, it serves as both local oscillator for coherent receiver and transmitter in a Half-Duplex transmission mode. When two lasers are equipped, they can work in Full-Duplex transmission mode as receiver or transmitter indistinctly and back-up protection is provided in case of laser fail. Finally, a simple activation process based on Half-Duplex transmission for both lasers and a wavelength monitoring during operation is also proposed.